Hydraulic drive motors have been used on various automotive and non-automotive vehicles for years. Hydraulic wheel motors have achieved a fair degree of commercial success in farm and off road vehicles, as shown in U.S. Pat. No. 3,584,698, assigned to Caterpillar Tractor Company. A number of hydraulic wheel motors are shown in the prior art, such as those disclosed in U.S. Pat. Nos. 2,418,123, 3,179,016, 3,280,934, 3,584,698 and 3,612,205. These hydraulic wheel motors are provided with a rotary member and a stationary member having a plurality of cavities for pumping hydraulic fluid which periodically varies in displacement as the stationary and the rotary member move relative to one another. Hydraulic fluid from an engine driven pump is supplied to the hydraulic wheel motors for propelling the vehicle.
Finney, in U.S. Pat. No. 3,747,722 first suggested the use of pumps cooperating with the mechanically driven wheels of a vehicle to hydraulically drive hydraulic wheel motors coupled to the non-mechanically driven wheels. The pumps and motors are attached to the vehicle outboard of the axle and brake drum, thereby increasing the vehicle's track by two times the pump thickness. The Finney system coupled front and rear wheels on the same side of the vehicle together hydraulically.
An object of the present invention is to provide a hydraulic drive apparatus to convert a two wheel drive to a four wheel drive vehicle with minimal alteration to the vehicle, and without increasing the vehicle track.
Another object of the present invention is to provide a hydraulic four wheel drive mechanism which can be utilized at low speeds and disengaged at high speeds to eliminate friction loss.
Yet another object of the invention is to provide independently controllable hydraulic wheel motors or pumps adjacent each wheel which can be regulated to maximize tractive force in steering maneuvers.
These and other objects, feature and advantages of the present invention will be more fully understood with reference to the drawings, specification, and claims.